IDENTIFICATION OF THE TYROSINE PHOSPHATASE PTP1C AS A B-CELL ANTIGEN RECEPTOR-ASSOCIATED PROTEIN INVOLVED IN THE REGULATION OF B-CELL SIGNALING

Recent data implicating loss of PTP1C tyrosine phosphatase activity in the genesis of the multiple hemopoietic cell defects found in systemi c autoimmune/immunodeficient motheaten (me) and viable motheaten (me(v )) mice suggest that PTP1C plays an important role in modulating intra cellular signaling events regulating cell activation and differentiati on. To begin elucidating the role for this cytosolic phosphatase in ly mphoid cell signal transduction, we have examined early signaling even ts and mitogenic responses induced by B cell antigen receptor (BCR) li gation in me and me(v) splenic B cells and in CD5+ CH12 lymphoma cells , which represent the lymphoid population amplified in motheaten mice. Despite their lack of functional PTP1C, me and me(v) B cells prolifer ated normally in response to LPS. However, compared with wild-type B c ells, cells from the mutant mice were hyperresponsive to normally subm itogenic concentrations of F(ab')(2) anti-Ig antibody, and they exhibi ted reduced susceptibility to the inhibitory effects of Fc gamma IIRB cross-linking on BCR-induced proliferation. Additional studies of unst imulated CH12 and wild-type splenic B cells revealed the constitutive association of PTP1C with the resting BCR complex, as evidenced by cop recipitation of PTP1C protein and phosphatase activity with BCR compon ents and the depletion of BCR-associated tyrosine phosphatase activity by anti-PTP1C antibodies. These results suggest a role for PTP1C in r egulating the tyrosine phosphorylation state of the resting BCR comple x components, a hypothesis supported by the observation that PTP1C spe cifically induces dephosphorylation of a 35-kD BCR-associated protein likely representing Ig-alpha. In contrast, whereas membrane Ig cross-l inking was associated with an increase in the tyrosine phosphorylation of PTP1C and an similar to 140-kD coprecipitated protein, PTP1C was n o longer detected in the BCR complex after receptor engagement, sugges ting that PTP1C dissociates from the activated receptor complex. Toget her these results suggest a critical role for PTP1C in modulating BCR signaling capacity, and they indicate that the PTP1C influence on B ce ll signaling is likely to be realized in both resting and activated ce lls.